Plants are under almost constant attack by bacterial and fungal pathogens in the environment and economic losses are a frequent consequence of this assault. To avoid or reduce economic loss, synthetic chemical fungicides have been traditionally used to keep the development of disease in check. However, the impact of chemical pesticides on the environment and human health can be harmful; this impact has been well-documented. Improperly used chemical pesticides can contaminate water, air, and soil and can have lasting harmful effects on aquatic life, birds, mammals, and beneficial insects such as bees. Further, pest populations can become resistant to many chemicals requiring the use of higher doses to obtain effective control. As a result, there is increased demand for safe and effective alternatives to chemical pesticides.
Pathogens in the genus Phytophthora are known to be causative agents for numerous plant diseases. Sudden oak death, soybean root rot, apple crown and collar rot, and root rots affecting American chestnuts, rhododendron, African violet, and strawberries, are just examples of diseases caused by this group of pathogens. Typically, plant diseases caused by Phytophthora are very difficult to control and often lead to the death of the plant. This pathogen is widespread and an economic problem to growers around the world. For example, Phytophthora infestans was the infective agent of the potato that caused the Great Irish Famine between 1845 and 1849. Attempts to manage Phytophthora spp. with techniques such as rootstock selection and site modification have been mostly unreliable.
Fungi in the genus Pythium are commonly called water molds and many Pythium spp. are plant pathogens of economic importance in agriculture. Pythium damping off is a very common problem in fields and greenhouses where the organism kills newly emerged seedlings (Jarvis, W. R., “Managing Diseases in Greenhouse Crops,” APS Press, St. Paul, Minn. (1992)). Pythium spp. tend to be very generalistic and unspecific in their host range. They infect a large range of hosts (Owen-Going, T. N., “Etiology and Epidemiology of Pythium Root Rot in Bell Pepper (Capsicum annuum L.) in Commercial-Scale and Small-Scale Hydroponic Systems,” M. Sc. thesis, University of Guelph, Guelph, Ontario (2002)), while Phytophthora spp. are generally more host-specific. For this reason, Pythium spp. are more devastating to commercial crop production because crop rotation alone will often not eradicate the pathogen (nor will fallowing the field, since Pythium spp. are also good saprotrophs, and will survive for long periods of time on decaying plant matter).
Fusarium is a large genus of filamentous fungi widely distributed in soil and in association with plants. Most species are harmless saprophytes and are relatively abundant members of the soil microbial community. However, there are a number of important Fusarium species that are pathogenic to agricultural crops. For example, some species produce mycotoxins in cereal crops that can affect human and animal health if they enter the food chain. The genus includes a number of economically important plant pathogenic species, including F. graminearum that can infect barley, particularly if there is rain late in the season. It is of economic impact to the malting and brewing industries as well as feed barley. Fusarium contamination in barley can result in head blight and, in extreme contaminations, the barley can appear pink. Fusarium graminearum can also cause root rot and seedling blight. Diseases caused by Fusarium spp. can be economically devastating. For example, total losses in the U.S. of barley and wheat crops between 1991 and 1996 from F. graminearum are estimated at $3 billion.
Rhizoctonia spp. are among the most diverse of plant pathogens, causing root, stem and foliar diseases of many of our most important herbaceous and woody ornamentals. Rhizoctonia spp. usually attack plants at the soil line, causing root loss and constriction of the stem which results in girdling and death of the tops. This pathogen can attack leaves as well, and is especially severe when plants are grown close together and kept moist.
Thielaviopsis is a small genus of fungi that includes several important agricultural pathogens. The most widespread is T. basicola, the causal agent of root rot diseases in crops including cotton and a variety of vegetables. In cotton, Thielaviopsis root rot, also known as black root rot, causes necrosis of the roots and stunting of the crop plants.
Powdery mildew is a disease that attacks a wide range of host plants, including cereals and grasses, vegetables, flowers, shrubs, fruit trees, and broad-leaved shade and forest trees. Although powdery mildew appears similar on most host plants, it is caused by a large number of different fungal pathogen genera and species. The disease is characterized by a white or grayish growth on the surface of leaves, stems, flowers, and fruit. It is generally not a fatal disease, but can cause leaf curling and yellowing, stunting of the host plant and defoliation. It is particularly detrimental to ornamental plants, significantly decreasing their value and marketability due to unsightly symptoms.
Downy mildew is a broad category of diseases that can appear on many host plants and is particularly devastating on crucifers, grapes, hops, and many vegetable crops. Symptoms appear as small, green or yellow angular spots on the leaf surface that can spread to the entire plant. Infected plant parts eventually become brown and necrotic and can die.
Botrytis blight or gray mold is a fungal disease that infects a wide array of herbaceous annual and perennial plants. There are several species of the fungus Botrytis that can cause blights; the most common is Botrytis cinerea. Botrytis infections are favored by cool, rainy conditions—this disease is particularly damaging when rainy, drizzly weather continues over a period of several days.
Bacterial diseases of plants occur in almost every environment and can be extremely destructive. Their significance as pathogens is based largely on the fact that they can multiply very quickly and produce large numbers of cells in a short period. Bacteria enter through wounds, stomata or other natural openings and can be transmitted in water, through the air, and by contaminated equipment, people, and vehicles.
Bacteria can cause a number of diseases on ornamental and agronomic crops. These include leaf spotting on English ivy, fireblight on apples and pears, crown gall on stone fruits, and wilts in geraniums and cucurbits (cucumbers, squash and melons). Common plant pathogenic bacteria include Erwinia spp., Dickeya spp., Pseudomonas spp., Xanthomonas spp., and Clavibacter michiganensis. These pathogens may attack plant root systems, foliage, or a combination of both. Disease-free stock plants, resistant cultivars, and sanitation are used to help prevent bacterial diseases. The most common chemical sprays for bacterial diseases contain copper—these help to slow the spread of the disease. In general, chemical sprays for bacterial diseases are less reliable than for most fungal diseases.
The present invention is directed to overcoming these and other deficiencies in the art.